Maintaining high levels of dry strength is a critical parameter for many papermakers. Obtaining high levels of dry strength may allow a papermaker to make high performance grades of paper where greater dry strength is required, use less or lower grade pulp furnish to achieve a given strength objective, increase productivity by reducing breaks on the machine, or refine less and thereby reduce energy costs. The productivity of a paper machine is frequently determined by the rate of water drainage from a slurry of paper fiber on a forming wire. Thus, chemistry that gives high levels of dry strength while increasing drainage on the machine is highly desirable.
Glyoxalated acrylamide-containing polymers are known to give excellent dry strength when added to a pulp slurry.
U.S. Pat. No. 5,938,937 teaches that an aqueous dispersion of a cationic amide-containing polymer can be made wherein the dispersion has a high inorganic salt content.
U.S. Pat. No. 7,323,510 teaches that an aqueous dispersion of a cationic amide-containing polymer can be made wherein the dispersion has a low inorganic salt content.
European Patent No. 1,579,071 B1 teaches that adding both a vinylamine-containing polymer and a glyoxalated polyacrylamide polymer gives a marked dry strength increase to a paper product, while increasing the drainage performance of the paper machine. This method also significantly enhances the permanent wet strength of a paper product produced thereby. Many cationic additives, but especially vinylamine-containing polymers, are known to negatively affect the performance of optical brightening agents (OBA). This may prevent the application of this method into grades of paper containing OBA.
According to U.S. Pat. No. 6,939,443, the use of combinations of polyamide-epichlorohydrin (PAE) resins with anionic polyacrylamide additives with specific charge densities and molecular weights can enhance the dry strength of a paper product. However, these combinations also may elevate the wet strength of the resultant paper to the point that repulping broke paper is extremely difficult and inefficient.